Colicin D

Colicin D is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them with its tRNase activity; it digests specific tRNAs, inhibiting protein synthesis and leading to the death of the cell.

Synthesis and release
The operon for colicin D is encoded on a plasmid in the cytoplasm of the E. coli. This operon also encodes its Colicin Immunity Protein, ImmD, to protect the cell from the tRNase activities of the colicin, alongside a protein to aid the release of the colicin outside the cell.

Once produced, the immunity protein binds to the cytotoxic domain of the colicin and inhibits its enzymatic activity.

Mechanism of uptake
The receptor binding domain of the colicin binds to the outer membrane receptor FepA on the target cell; it is a constitutively expressed protein parasitised by the colicin. The translocation domain then recruits proteins from the Ton system to translocate the protein across the membrane and into the cytoplasm.

Killing Activities
Colicin D kills its target cell through its cytotoxic domain which carries tRNase activity; it cleaves the anticodon of select tRNA molecules, preventing them from being paired with the codon when in the ribosome. This halts protein biosynthesis, ultimately leading to the death of the cell.